So, yes, there's already a library called glfx.js which basically does the same thing. And I'm almost saddened to say that it's excellent.

It's not all in vain, however. My implementation features raw convolution and color matrix filters. The latter should be particularly interesting for those coming from the Flash world: it's the exact same as Flash's ColorMatrixFilter and allows for some nice "Instagrammy" photo effects among others. There are some JavaScript libraries around that implement this ColorMatrixFilter, but they all do it in JavaScript directly, which is quite slow. Mine can be used in realtime, even on iOS with Ejecta.

I also learned some things about WebGL and Shaders with all this. One interesting aspect is that you can't draw a framebuffer texture onto itself. So in order to apply more than one effect for the same image, you need to juggle around 2 textures - use one as the source, the other one as the target and then switch. And if you don't want to waste any draw calls, you need to know which draw call will be the last, so that you can draw directly onto the target Canvas instead of on a framebuffer texture.

WebGL's coordinate system also complicates things a bit. It has the origin in the bottom left corner, instead of in the top left like most 2D APIs, essentially flipping everything on the Y axis. This has caused me a lot of pain with Ejecta already. The solution sounds trivial: just scale your drawing coordinates by -1, but this only gets you so far. If you need to get the pixel data of your image or draw one canvas into another, you suddenly have to invert everything again.